I'll answer what i can , this thread doesn't see much action anymore so i hope i don't steer you wrong .
I'm just wondering if it might be possible to try a test piece before starting , see how the...

hmmm - you could put a wood strip between the two bars & pour the bar addition seperately . or if you dont want to see the strip , you could pour the bar addition , allowing enough etex to flow over...

There shouldn't be yellowing issues . Scratches can be waxed or compounded out . As extra protection you can put a coat of wax on the Etex .
You can't put hot pans on the surface . Also if anything...